This invention relates to telecommunications systems and more specifically in a cable modem system to a method and apparatus for reducing the time it takes to ascertain that a modem has failed.
Modem cable television systems are widely used not only for the transmission of television signals to subscriber homes, but also for the two-way transmission of data to and from subscriber homes. A cable modem network consists of a number of cable modems in subscriber homes, a cable modem headend system, and a coaxial or hybrid-fiber/coax communication link between the headend and the subscriber modems. Unlike traditional dial modems, cable modems transmit and receive digital signals at radio frequencies.
Prior to the present Data Over Cable Service Interface Specification, or DOCSIS, a number of cable modem and headend system vendors developed a wide range of systems which worked differently, with each having their own unique proprietary protocol for transmission of data over the communications link. As a result of this ad hoc approach, standardization was deemed necessary and appeared to be required so that cable company operators could obtain competitive pricing. The benefit was to be able buy to a modem from any vendor and make sure that it would work with any headend system. A standard protocol was subsequently issued by the DOCSIS standards group, which developed the standard now known as the DOCSIS RF interface specification.
As part of the DOCSIS specification, there is a protocol relating to ranging. Ranging refers to the ability of a headend system to measure power level, frequency and timing of signals generated by a subscriber modem and transmitted over a communication link; and to send error or correction signals back to the subscriber modem so as to optimize the subscriber modem for robust data transmission. These error signals are then used by the subscriber modem to adjust the power level, to modify the frequency, and to transmit data with adjusted timing.
According to the DOCSIS protocol, an invitation to range is sent to each of the subscriber modems. The subscriber modems then send a ranging request to the headend system. The headend system then sends a map message to the subscriber modems. Ordinarily the map message includes information regarding the upstream bandwidth allocation and which modem can transmit, and when. The typical items that might be included in the map message are opportunities for modems to ask for bandwidth, opportunities for modems to transmit data, opportunities for modems to join the network, and invitations to send ranging requests.
In terms of the DOCSIS standard, it is required that the headend system provide one invitation for each modem to range no less than once every thirty seconds. If thirty seconds elapse during which time the modem fails to receive a ranging invitation the modem is required to reset. Because of the potential for losses on the downstream communications link, it is important to provide more than one invitation every thirty seconds. Moreover, it will be appreciated that this requirement to transmit a ranging request at least every thirty seconds in some instances is too infrequent for the receiver to reliably receive the invitation. This is because of losses along the communication link. It is therefore important to have ranging invitations transmitted more frequently, such as every ten seconds.
Thus, because of the possibility of lost messages, it is oftentimes required that the subscriber modem range for instance every ten seconds as opposed to every thirty seconds. This utilizes not insignificant resources, both at the subscriber modem and at the headend system. Note, the amount of signaling between the subscriber modem and the headend system increases as ranging invitations increase in rate.
Assuming that the subscriber modem is sent a ranging invitation every ten seconds, and further assuming from the DOCSIS specification that a modem is ascertained as having failed if to there is no response from the modem after sixteen tries, then it will be appreciated that it may take one hundred and fifty seconds to ascertain that the modem has failed, ie. two and a half minutes.
From an operational point of view, this is an exceedingly long period of time to ascertain that the modem is not responding.
The alternative to improving this scenario is to poll the modems somewhat more frequently. However, if for instance the modems are polled once a second, the overhead necessary to support such a polling function of for instance 1500 modems is excessive. Were this the case, bandwidth limitations would restrict the amount of data transmittable by the modem to the headend system and vice versa due to the high overhead involved. For instance, assuming 1500 modems are linked to a particular headend system, and further assuming that each of the modems is being invited to range once a second, up to 100 percent of the bandwidth is used in the process which either eliminates data transmission altogether or significantly reduces the amount of the data through-put and slows down communications.
There is therefore a need to be able to ascertain in the quickest possible time that a modem has failed, while not significantly using up the resources of the system.
In order to reduce the time that it takes to ascertain that a modem has failed, in the subject invention the frequency at which periodic invitations are transmitted to the subscriber modem is switched from a low frequency to a higher frequency upon the detection of a predetermined number of ranging nonresponses. In one embodiment, ranging invitations are sent out at a low rate or frequency, for instance every ten seconds. If the subscriber modem does not respond after three invitations, the periodic invitations are sent out a higher rate or frequency, in one embodiment once every five seconds.
The result is that the time it takes to ascertain that a modem has failed is decreased based upon a previous, albeit short history of failures. Thus for instance the sixteen failed attempts specified by the DOCSIS protocol can be ascertained within 1xc2xd minutes, instead of within 2xc2xd minutes. The time for ascertaining modem failure of course depends upon the number of ranging nonresponses which constitute the trigger point and the increased rate or frequency at which ranging invitations are transmitted.
It will be appreciated that in the illustrative example, while the higher frequency is set to be twice that of the low frequency, it is possible to configure the system to even further reduce the time necessary to ascertain modem failure by either increasing the higher frequency or decreasing the number of initially failed responses which result in switching to the higher frequency. Thus, the time to ascertain modem failure can be reduced even further.
From the headend system administrator""s point of view, the subject system permits an increase in upstream bandwidth to the extent that overhead and resources are not over-utilized by the ranging process. By upstream is meant the direction that data is sent from the subscriber modem to the headend system. Thus, in the subject system the headend system is spending less time performing the ranging task, thereby freeing up resources at the headend to permit the forwarding of data or packets.
In summary, in a cable modem system a method and apparatus is provided to reduce the time to ascertain modem failure by sensing ranging nonresponses resulting from periodic ranging invitations and increasing the frequency of the ranging invitations upon a predetermined number of nonresponses. The increased frequency of ranging invitations decreases the time required to ascertain modem failure. Modem failure is defined to be a predetermined number of nonresponses, for instance 16 nonresponses. In so doing, a system is provided which minimizes the bandwidth necessary for rapid modem failure detection. This is accomplished by increasing the frequency of invitations when a predetermined number of failures are detected, thus minimizing the number of resources necessary for modem failure detection as well as reducing bandwidth requirements.